Some guys do this naturally, but for most of us - using body language correctly or incorrectly can mean the difference between her feeling attraction (she won't be sure why) or her being not attracted.
A LOT of this is subconscious - we don't even realize it is happening - so it must be buried deep in our psyche - in the primitive part of our brain.
Have you ever felt really uncomfortable when someone was standing near you, but you were not sure why? Have you ever felt very comfortable and had great rapport with a stranger, and still not sure?
Let me give you a hint, it almost always has nothing to do with the conversation or your looks. It has to do with the subconscious signals our bodies are receiving. 90% of human interaction is non-verbal.
For example, if a girl is facing away from you and you are facing directly at her - she will feel a strange feeling that you are coming on very strong and whenever someone feels that, it is natural for them to pull away. This is why she 'just feels' awkward about you.
Other times, if she is facing you and you are facing away, she feels like she is trying to hard and you are ignoring her. Since women typically have a self esteem that is easily bruised (well we all do), she will unconsciously feel like you are not into her and she will feel awkward and not enjoy the conversation.
Establishing rapport is difficult in any of these situations.
But what if you you are both standing side by side, talking almost over your shoulder in a bit. That is a very common stance for people who are friends and watching what is happening in the world around them.
THIS NATURALLY IS COMFORTABLE, NORMAL AND WHAT PEOPLE WITH RAPPORT DO WITHOUT THINKING. It just feels good and neither person feels that the other is trying to hard and it makes you want to continue talking to that person.
MOST guys do not do this. They are standing there too close to the girl and being super needy with their body language by facing a girl who is not facing them. DO NOT BE THAT GUY!!!
Instead, stand side by side and talk. As you generate rapport, she will turn towards you without even thinking about it.
NOW THIS IS KEY. You MUST turn to face her or she will feel awkward (again not realizing why). So now you can face her and continue your rapport building.
You both will feel comfortable, once again without realizing it.
Your conversation matters much, much less than what your body language is saying.
That is why many pickup lines do not work. Body language is arguably 10x more important than what is said.
Try this yourself... even with a friend and you will just FEEL the difference.
Best of luck.